1. Field of the Invention
The present invention relates to the generation of white light pulses and the wavelength conversion of light pulses using a nonlinear optical device.
2. Description of the Related Art
Heretofore, it has been known to generate white light pulses by introducing light pulses into an optical fiber such as a highly nonlinear optical fiber or a photonic crystal optical fiber (also called a holey fiber) to generate supercontinuum light (While an applied light pulse is propagating through a medium, it is subjected to self-phase modulation due to a nonlinear optical effect, and its spectrum is continuously spread in a very wide wavelength range. The light pulse with the spread spectrum is called supercontinuum light). The wavelength conversion of light pulses is realized by extracting light pulses having a desired wavelength from supercontinuum light with a tunable optical filter. Details of such a process are disclosed in William J. Wadsworth, Arturo Ortigosa—Blanch, Jonathan C. Knight, Tim A. Birks, T. P. Martin Man, and Phillip St. J. Russell, Journal of Optical Society of America B, Vol. 19, No. 9. pp. 2148-2155 (2002) (first document).
FIG. 1 of the accompanying drawings shows a cross-sectional SEM (Scanning Electron Microscope) photographic representation of a typical holey fiber. As shown in FIG. 1, the holey fiber has a central core having a diameter of about 2 μm, and includes many holes in a cladding around the core. The core is made of silica (SiO2) and has a refractive index of about 1.5. The cladding is also made of SiO2 and has a refractive index of about. 1.0 because of the holes formed therein. The difference between the refractive indexes of the core and the cladding is about 0.5. In the holey fiber, because light pulses are confined in and propagated through the small core having the diameter of about 2 μm, the light pulses strongly interact with the silica that the core is made of, developing a large nonlinear optical effect. Supercontinuum light is generated based on such a large nonlinear optical effect. By designing the holey fiber such that dispersion of light is essentially reduced to zero at the wavelength of the light pulses, the light pulses are prevented from being spread while being propagated through the optical fiber, making it possible to generate supercontinuum light efficiently.
The conventional process of generating supercontinuum light with an optical fiber such as a holey fiber requires the optical fiber to have a length of at least several centimeters in order to provide a sufficient interaction length. Since the generation of supercontinuum light needs light pulses having a peak power in kW to MW ranges and a pulse duration of several hundred fs, a large-size solid-state laser such as a titanium-sapphire laser has to be used as a pulse light source. Optical fibers such as holey fibers have no or very little dependency of propagation characteristics on propagated light having perpendicular planes of polarization. Therefore, if the optical fiber is bent or twisted, the planes of polarization are rotated while the light is being propagated through the optical fiber. If the optical fiber is long, such a phenomenon manifests itself, posing a problem on the stability for generating supercontinuum light.
As described above, the conventional process of generating supercontinuum light with an optical fiber such as a holey fiber is problematic in that a pulse light source and a wavelength-variable pulse light source comprising a holey fiber are large in size, and the generated supercontinuum light lacks stability. Furthermore, large-size solid-state lasers are highly expensive, and holey fibers are also highly expensive at present (several ten thousands yen per meter).
It has been studied to ascertain whether other nonlinear optical devices than highly nonlinear optical fibers and holey fibers may be used to generate supercontinuum light or not. One candidate for such other nonlinear optical devices is a thin-wire optical waveguide.
FIG. 2 of the accompanying drawings shows an example of a thin-wire optical waveguide. As shown in FIG. 2, the thin optical waveguide has substrate 20 such as of a semiconductor, lower cladding 21 of silica (SiO2) disposed on substrate 20, and core 22 in the form of a thin silicon (Si) wire disposed on lower cladding 21. Core 22 has entrance end facet 220 and exit end facet 221 on its respective opposite ends. Prototype thin-wire optical waveguides having cores 22 whose heights range from 0.2 to 0.25 μm and whose widths range from 1.0 to 0.5 μm have already been fabricated. Since core 22 is made of Si, the refractive index thereof is about 3.5. Lower cladding 21 is made of SiO2 and has a refractive index of about 1.5. An upper cladding is provided by air and has a refractive index of 1. Therefore, the difference between the refractive indexes of the core and the claddings is at least about 2. The thin-wire optical waveguide thus has a large refractive index difference between the core and the claddings.
FIG. 3 of the accompanying drawings shows an example of another thin-wire optical waveguide (see Japanese laid-open patent publication No. 2003-322737). As shown in FIG. 3, the thin-wire optical waveguide has substrate 30 such as of a semiconductor, lower cladding 31 of SiO2 disposed on substrate 30, and core 32 in the form of a thin Si wire disposed on lower cladding 31. Unlike the thin-wire optical waveguide shown in FIG. 2, the above assembly is embedded in upper cladding 33 made of a polymer or SiO2. Core 32 has entrance end facet 320 and exit end facet 321 on its respective opposite ends. Core 32 has a cross-sectional size represented by a height in the range from about 0.2 to 0.3 μm and a width in the range from about 0.27 to 0.33 μm. Since core 32 is made of Si, the refractive index thereof is about 3.5. Lower cladding 31 and upper cladding 33 are made of SiO2 or a polymer and have a refractive index of about 1.5. Therefore, the difference between the refractive indexes of the core and the claddings is at least about 2. The thin-wire optical waveguide shown in FIG. 3 thus has a large refractive index difference between the core and the claddings.
Since the core of each of the thin-wire optical waveguides shown in FIGS. 2 and 3 is constructed as a thin wire, a large coupling loss is caused when light is introduced into the thin-wire optical waveguide. For increasing the efficiency with which to introduce light into the thin-wire optical waveguide, a process of forming a beam spot size converter by covering a tip end of the thin-wire optical waveguide with a polymer optical waveguide is disclosed in T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, Electronics Letter, Vol. 38, No. 25. pp. 1669-1670 (2002) (second document).
The thin-wire optical waveguides described above is capable of realizing stronger light confinement than holey fibers. However, thin-wire optical waveguides have not widely been used as a nonlinear optical device. It has not been proposed to design the structure of a thin-wire optical waveguide which is suitable for generating white light pulses in order to achieve a light confining structure which is efficient to guide light pulses through a thin-wire optical waveguide and generate white light pulses, to design the structure of a thin-wire optical waveguide for controlling the dispersion of light, and to design the structure of a thin-wire optical waveguide for controlling planes of polarization for stabilizing white light pulses.
As described above, a thin-wire optical waveguide causes a large coupling loss when light is introduced into the thin-wire optical waveguide because the core is thin. It has not been studied to find a way to solve this problem in the generation of white light pulses.
Another nonlinear optical device which can possibly be used to generate supercontinuum light is a photonic crystal optical waveguide as shown in FIG. 4 of the accompanying drawings. However, it has not been proposed to design the structure of a photonic crystal optical waveguide for efficiently generating white light pulses and to design the structure of a photonic crystal optical waveguide for controlling the dispersion of light.